Does duct size really matter when using ECM2 motor?

Recently replaced my forced air oil furnace with a 5 ton dual speed
WaterFurnace Envision. My HVAC guy told me the not to worry about my
ducting as the ECM motor will overcome the lack of duct size. I mainly
concerned about my return air vents.
Here are some details;
- 2 story house - 2800 sq/ft
- Near Toronto Ontario
- Two supply ducts 12"x8" and 12"x7"
- Two return duct 14"x6" and 12"x8"
- 18 heat registers feed by 5" dia ducts
- 4 return air vents 15"x7" (2 up stairs, 2 downstairs)
- Design heat loss of 63,312 btu/h
- Heat gain 30,000 btu/h
- Furnace blower settings - 1st stage CFM00, 2nd stage CFM50
Quite honesty everything seems to be working fine...... slight noise
from downstairs return vents, however we're warm. Defiantly much long
run times over oil.
I've read many times the CFM should be 400 per ton (cooling?)..... I'm
nowhere close. So I having trouble sleeping at night not know if I my
ducting is incorrect or I shouldn't worry as the ECM motor overcomes it
by pushing more air. Also adding to my confusing is the supply duct on
the furnace measures only 14"x14".
I'd very much appreciate your thoughts and advice,
Thanks

Yes you can increase air flow by increasing motor size
but you also increasing air friction the air friction also
generate heat so you also increasing load on AC system
it is needles to say that your electric bill goes up too
You can not get something for nothing
From Dido
I mainly

400 CFM per ton is a good average, but better still is a proper Manual J and
Manual D calculation to correctly size the system and the ductwork. From
what you have posted, you need to have your duct system ripped out, and
correctly sized/designed and re-installed.

your hvac guy is a clueless brain dead moron for giving you such
false advice.

airflow is dependent upon the restrictions imposed by friction losses
of the duct system and obstructions such as filters and air
conditioning coils. also, the grills themselves impose restrictions
to the air flow.
to achieve the nominal 400 cfm per ton, its obvious you cant change
the coil, although you can change air filtration and ducting. but why
sacrifice air filtration because your hvac tech is brain dead?
the solution is to design the duct system (both supply and return) so
that it imposes a very nominal total static pressure on the fan motor.
now a variable speed blower will help overcome some of these
restrictions, but it will not correct a faulty design. and it will
cost you more money in operational costs than if the duct system was
designed correctly from the git-go.
my ductulator says that if i want to move 1950 cfm at .08" wc, my
round duct size must be 19" OR a 20 x 16 duct. OR two 15" round
ducts.
why .08" wc? well, the maximum your fan will deliver is in the .5-.7
range. Now add in a wet coil factor of .35 to .45, throw in a dirty
filter or whatever, then the grills, and whats usually left is a
number under .1 for the duct system. there are different ductulators
for metal or flex ducts. A smart ccokie will design around the .08
figure and be very very close 99.9% of the time.
my suggestion is to find somebody who knows how to properly size air
distribution systems.

Dam it! I was afraid to hear that! Thanks for your time and
suggestion.....
When it was suggested "someone who knows distribution systems" is this
referring to another HVAC technician or an Engineer? What do I look for
in the Yellow pages?
Was that one 20 x 16 duct for supply and another 20 x 16 for return?
I'd prefer to have the duct system designed and do the work myself as I
have full access to duct work...... and no money.
I'm still puzzled why the furnace has only a 14"x14" supply duct;
however it suggested I'll need a 20" x 16"?
Please bare with me, I'm really trying to understand how all this works
so I won't make that same mistake twice.
Thanks again for your suggestions.
snipped-for-privacy@gonefishin.net wrote:

yup. total airflow in the supply typically = total airflow in the
return. sometimes the return can be decreased by 10% if there is that
much fresh air being brought in thru a dedicated duct from the great
outdoors. 20% or more on commercial systems.

no problem. all you need to do is obtain a copy of ACCA Manual D
Residential Duct Systems. Read it until you comprehend what it is
saying, and then proceed. remember, this is not rocket science.
anybody that can read & comprehend english can do this work.
errrr well most everybody.....
http://cgi.ebay.com/Residential-Duct-Systems-Manual-D-BOOK-ACCA-ANSI_W0QQitemZ270080735569QQihZ017QQcategoryZ2228QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

nobody can see your system except you. I have no idea what you need.
However, if I want to move 1950 cfm at .08" w.c., I will need a 16x20
duct to do so. Actually a 15x20, but I like even numbers.
why is it 14 x 14? which is cheaper to build? who got to do the
original install? the lowest bidder. now you know.

Thanks Gone Fishing.... you really gave me hope! I'll order manual D
today.
You can see what I mean about the 14"x14" supply duct in the
Waterfurnace manual, page 13, here's a direct link:
http://www.waterfurnace.com/Envision/pdf/SP1585.pdf
- horizontal model 060-070
- discharge connection "M" - 13.6"
- discharge connection "N" - 13.2"
FYI, here's a very user-friendly Design Heat Loss program that does
duct sizing too;
http://www.hvaccomputer.com
In the mean time I'd like to increase my 4 air return vents by removing
some drywall to open up the holes. Can you kindly suggest the grill
sizes required keeping in mind I'll increase the actual return ducting
in the near future.
Thanks again for your help, I'd really like to but ya some beers.
snipped-for-privacy@gonefishin.net wrote:

what you're looking at on that page is the discharge opening of the
blower. the airhandler is rated at 5-6 tons, so consequentially, yes,
it will have a discharge of that size, based soley on the blower
wheel. Notice the return air opening size? Ideally your supply duct
would be somewhat similiar, based on the total cfm YOUR system is
supposed to be delivering. If you maintain only the 14x14 supply
duct, the static pressure inside will be such that it limits the
ability of the blower to deliver the correct airflow, plus it will
cost you $$ in operational costs.
My ductulator says at 1950 cfm a 14x14 duct by itself will impose a
.2" w.c. static pressure on the blower. Now factor in all the other
restrictions like filter, coil, & too small return ducting. You ARE
NOT getting the engineered airflow the manufacturer of your equipment
recommends. It cannot perform to factory specs.

I learned correct duct sizing long before there were computer
programs. Computer programs exist so people dont have to learn the
basics. Any moron can use a program to plug in numbers. Computer
programs that are based on ACCA Manual J & D are approved by city
building departments. I would strongly question software that wasnt
approved.
You dont need software if you can read and comprehend ACCA Manual D.
FWIW, the ONLY software I would recommend is
http://www.wrightsoft.com/overview/unitedstates/default.asp?country=US

yes. install 4 each 14x20 bar type commercial grade return grills.
From these run 4 12" dia ducts back to the air handler. The 14" is
width, they will fit inbetween the 16" on center wall studs. The 20"
is the vertical height. The 14x20 bar type grill will have the bars
running horizontally, parallel with the floor.

IF all the parameters are set up correctly.... I've found many quotes that
didn't have the numbers right...
The old rule of thumb for residential has always worked. Cheap customer .1"
Customer who desires quiet, comfort and quality .08"
However, on a two stage system such as Waterfurnace, if you use too large a
duct you better make sure your spread and throws on your registers are
matched also for the lower CFM if wall or ceiling mounted. Also, the larger
duct should be insulated or the end of the line will have cooler air coming
out. I like to keep my velocities up a little on two stage units which
allow me to use registers to match the low and high limits of the design. I
have gone as high as .13" depending on the size of the low stage for AC
verses the high side heat output.
Now, in defense of the contractor, Waterfurnace and everyone else who is a
supplier has told me the exact same thing, "Don't worry about the duct size,
the ECM motor will overcome the resistance." I've heard this at least a
dozen times from Trane, American Standard, FHP, ClimateMaster, WaterFurnace,
Rheem, Rudd and ComfortAir. They are right up to .15" but after that you
will definitely have problems.
My biggest concern for this customer is the cfm over the resistant heaters.
I'm sure if a performance/cfm check was done you would find your lacking and
getting very close to cycling on overloads. And, if they insulated the duct
near those heaters you may have a fire just around the corner.
I'm not trying to scare you, just telling you the truth. I never put
insulated duct within 2 feet of electric resistant heaters unless I make a
shield on the inside that has 1" clearance from the insulation. Just this
week we put two Waterfurnaces in. The outlet was 13.5 x 13.5 We had a duct
14" square by two feet lining the actual insulated duct of 18 x 18. The
radiant heat will not be allowed to start a fire this way. Even insulating
the outside has potential problems. I have seen a fan fall and catch
insulation on fire and burn out a canvas connector.
You need a whole house Manual J and Manual D performed. Let a competent
dealer do this for you. Tell him your coming back here to make sure his work
is right.

at what cost to the end user? Note that these companies are not
paying the utility bill.
their perspective is ....screw the idea of proper design, lets let an
ecm motor overcome these shortcomings.....

Log in

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.